The invention relates to methods of converting diffused black-and-white images to lower spatial resolutions for transmission to remote facsimile devices that support only such lower spatial resolutions.
Facsimile machines have become common business and household appliances. One reason for their widespread acceptance is that they use a well-known and firmly established data communications protocol, which is common to all different makes and models of facsimile devices.
Historically, facsimile machines have utilized a spatial resolution of about 200 dots per inch (DPI). This resolution was agreed upon at a time when available data transmission rates were much lower than they are today.
With the availability of higher data transmission rates, more recent facsimile machines have been equipped with the capability of scanning and printing images at higher resolutions such of approximately 300 DPI. Facsimile protocols have been extended to allow such enhanced resolution. Compatibility between facsimile devices supporting different resolutions has been ensured by providing a negotiation procedure within the facsimile communications protocol. As a result of this negotiation procedure, a sending device transmits a facsimile image at the highest resolution that is supported by both the transmitting device and the receiving device.
In many cases, the resolution is negotiated prior to scanning documents. In these cases, documents are subsequently scanned at the negotiated resolution.
Some facsimile machines, however, support a mode in which batch scanning is performed initially, with actual transmission delayed at the user""s option. This is useful, for example, when scheduling a facsimile transmission for a later time, to take advantage of cheaper telephone rates.
Such delayed transmission can be problematic in situations where the sending device supports a higher resolution than the receiving devices. In a situation such as this, the sending device will typically scan the document at a high resolution. If it turns out that the receiving device supports only a lower resolution, the previously scanned image is converted to a lower resolution. The problem is that this conversion can seriously degrade the quality of the resulting facsimile image. In fact, the converted image is often worse than if the document had been originally scanned at the lower resolution.
This result is brought about by the use of diffusion methods when originally scanning the document at the high resolution. When scanning, an image is initially recorded as an array of scaled values, representing shades of gray. For example, values might range from 0 to 255, representing shades of gray from white to black. However, facsimile protocols are designed to support only binary or black-and-white images, in which individual dots are represented by binary or xe2x80x9con/offxe2x80x9d values corresponding to either black or white. Thus, before sending an image facsimile, its scaled values are converted to on/off values. When storing an image for later transmission, it is desirable to store the on/off values rather than the scaled values in order to conserve memory.
In converting scaled image values to on/off values, many facsimile devices utilize some form of diffusion (such as linear diffusion or error diffusion) in order to represent shades of gray as patterns of dots. Although this sacrifices resolution, it results in a more aesthetically pleasing reproduction of the original image.
Although diffusion produces a more pleasing black-and-white image, it results in scattered dots that create problems when converting to a lower resolution image. A conversion to a lower resolution is usually performed by omitting dots. For example, conversion from 300 DPI to 200 DPI might be performed by omitting every third dot or by equating each dot of the lower-resolution image with its nearest binary neighbor in the original, higher-resolution image. When converting in this manner, the scattered dots created during diffusion can become more closely grouped, leaving noticeable image artifacts.
One way to avoid this problem is to store scanned images as grayscale images, and to delay diffusing the image until a desired transmission resolution has been negotiated. The disadvantage of this approach is that grayscale images require much more memory than binary images.
The invention described below allows images to be stored in binary format, and to be converted to lower resolutions as needed, without significant degradation in image quality other than what would be expected when going to a lower resolution.
In accordance with the invention, a facsimile device scans a grayscale image and diffuses it to create a binary, black-and-white image having an array of on/off values. To convert the image to a lower resolution, the image is treated as a grayscale image and linear interpolation is performed to determine grayscale values of a lower resolution grayscale image. The lower resolution grayscale image is then diffused to create a lower resolution black-and-white image.